Method for adjusting the degree of substitution with acetyl group of cellulose acetate

ABSTRACT

A process for adjusting an intermolecular or intermolecular degree of acetyl substitution of cellulose acetate is disclosed. The process comprises ripening cellulose acetate in the presence of a catalyst, an acetyl donor, and water or an alcohol. The amount of water and the alcohol is in the range of 0.1 to 10 mol % based on the amount of the acetyl donor.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority of Japanese Application No.PCT/JP02/02410 filed Mar. 14, 2002 which claims the priority of JapaneseApplication Nos. 2001-73076 filed Mar. 14, 2001; 2001-195843 filed Jun.28, 2001 and 2001-339692 filed May 11, 2001, the complete disclosures ofwhich are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a process for adjusting intermolecularor intramolecular degree of acetyl substitution by ripening celluloseacetate.

BACKGROUND OF THE INVENTION

Cellulose acetate, particularly cellulose acetate having a degree ofacetyl substitution of 2.6 or more (generally classified into cellulosetriacetate), is tough and excellent in heat-resistance. Therefore,cellulose acetate has been used in various technical fields. Forexample, a cellulose acetate film is a representative photographicsupport. Further, a cellulose acetate film shows an excellent opticalisotropy. Accordingly, a cellulose acetate film is used in a liquidcrystal display, which has recently extended its market. In the liquidcrystal display, the cellulose acetate film is used as a protective filmof a polarizing plate, a phase retardation film or a color filter.

A cellulose acetate film is generally formed according to a solvent castmethod, in which a cellulose acetate solution (dope) is cast onto asupport, and dried to evaporate the solvent to form a film.

In preparation of a cellulose acetate product such as the film accordingto the solvent cast method, a degree of acetyl substitution of thecellulose acetate is closely related to a solubility of the celluloseacetate in a solvent and physical characteristics (including opticalcharacteristics) of the products. It has been well known that celluloseacetate having a high degree of acetyl substitution shows a lowsolubility in a solvent, but forms a product of excellent physicalcharacteristics. On the other hand, cellulose acetate having a lowdegree of acetyl substitution shows a high solubility in a solvent, butforms a product having problems in physical characteristics.

Dichloromethene is the most conventional solvent of cellulose acetate.Cellulose acetate is well dissolved in dichloromethane. Therefore,cellulose acetate having a high degree of acetyl substitution has beendissolved in dichloromethane to prepare a cellulose acetate producthaving excellent physical characteristics according to a conventionalsolvent cast method. The low solubility of the cellulose acetate havinga high degree of acetyl substitution cannot be a problem wheredichloromethane is used as the solvent.

However, use of hydrocarbon halides such as dichloromethane has recentlybeen restricted severely to protect the global environment. Further,dichloromethane is apt to vaporize in the process for the preparation ofa product, because it has a low boiling point (41° C.). Accordingly,dichloromethane may cause problems in the working environment.Therefore, the process is conducted under closed conditions.

Japanese Patent Provisional Publication Nos. 9(1997)-95544,9(1997)-95557, 9(1997)-95538 propose a process for preparing thecellulose acetate solution, which comprises the steps of cooling amixture of cellulose acetate and an organic solvent, and then heatingthe mixture to prepare the solution. The process comprising the coolingand heating steps (which is sometimes referred to as a coolingdissolution method) makes it possible to prepare a solution fromcellulose acetate and an organic solvent in which cellulose acetatecannot be dissolved according to a conventional process. Therefore, thecooling dissolution method is very advantageous particularly where afilm is prepared from cellulose triacetate (having a degree of acetylsubstitution of 2.6 or more), which has poor solubility.

SUMMARY OF THE INVENTION

The cooling dissolution method now makes it possible to prepare acellulose acetate product from poorly soluble cellulose triacetate(having a degree of acetyl substitution of 2.6 or more) according to asolvent cast method without use of hydrocarbon halides such asdichloromethane.

However, cellulose acetate having a high degree of acetyl substitutionstill shows a low solubility in an organic solvent even if a coolingdissolution method is used. Further, a solution of cellulose acetatehaving a high degree of acetyl substitution in an organic solvent has aproblem in stability.

An object of the present invention is to improve solubility of celluloseacetate having a relatively high degree of acetyl substitution.

Another object of the invention is to provide a cellulose acetate inwhich the relation among the degrees of acetyl substitution at 2-, 3-and 6-positions is properly controlled.

A further object of the invention is to prepare a cellulose acetatesolution whose solubility and viscosity are easily controlled.

A furthermore object of the invention is to prepare a cellulose acetatefilm having preferable properties and optical characters.

A still further object of the invention is to prepare a celluloseacetate film having excellent physical characteristics by using asolution of cellulose acetate having a relatively high degree of acetylsubstitution.

The present invention provides a process for adjusting an intermolecularor intramolecular degree of acetyl substitution of cellulose acetate,which comprises ripening cellulose acetate in the presence of acatalyst, an acetyl donor, and water or an alcohol, and under acondition that the amount of water and the alcohol is in the range of0.1 to 10 mol % based on the amount of the acetyl donor.

The invention also provides a process for preparation of celluloseacetate which comprises the steps of: reacting cellulose in a solventwith acetic acid or acetic anhydride in the presence of an acid catalystto synthesize cellulose acetate; and ripening the synthesized celluloseacetate in the presence of the remaining acid catalyst, an acetyl donor,and water or an alcohol, and under a condition that the amount of waterand the alcohol is in the range of 0.1 to 10 mol % based on the amountof the acetyl donor.

The invention further provides a process for preparation of celluloseacetate which comprises the steps of: reacting cellulose in a solventwith acetic acid or acetic anhydride in the presence of an acid catalystto synthesize cellulose acetate; neutralizing the acid catalyst to stopthe synthesizing reaction; and ripening the synthesized celluloseacetate in the presence of a catalyst, and under a condition that theamount of water and an alcohol is less than 10 mol % based on the amountof the acetyl donor.

The above-mentioned processes can form cellulose acetate having newcharacteristics.

The invention provides cellulose acetate having a degree of acetylsubstitution in the range of 2.636 to 2.958, wherein the celluloseacetate shows a distribution curve of an intermolecular substitutiondegree in which the maximum peak has a half width (unit: difference ofintermolecular substitution degrees) of less than 0.080.

The invention also provides cellulose acetate having a degree of acetylsubstitution in the range of 2.636 to 2.958, wherein the celluloseacetate shows a distribution curve of an intermolecular substitutiondegree in which the maximum peak has a half width of less than Y definedin the following formula:Y=−0.17788X+0.5788in which X is the degree of acetyl substitution.

The invention further provides cellulose acetate showing an infraredabsorption spectrum, wherein the absorption spectrum has the absorptionmaximum in the wave number range of 3450 to 3550 cm⁻¹ in which theabsorption maximum has a half width of 135 cm⁻¹ or less.

The invention furthermore provides cellulose acetate in which thedegrees of acetyl substitution at 2-, 3- and 6-positions satisfy thefollowing formulas (I) to (III):2DS+3DS<6DS×4−1.70  (I)2DS+3DS<−6DS×4+5.70  (II)2DS+3DS>1.80  (III)in which 2DS is the degree of acetyl substitution at 2-position; 3DS isthe degree of acetyl substitution at 3-position; and 6DS is the degreeof acetyl substitution at 6-position.

The invention still furthermore provides a cellulose acetate in whichthe degrees of acetyl substitution at 2-, 3- and 6-positions satisfy thefollowing formulas (III) to (V):2DS+3DS>1.80  (III)3DS<2DS  (IV)6DS>0.80  (V)in which 2DS is the degree of acetyl substitution at 2-position, 3DS isthe degree of acetyl substitution at 3-position, and 6DS is the degreeof acetyl substitution at 6-position.

According to study of the present inventors, the intermolecular degreeof acetyl substitution can be adjusted to be uniform by ripeningcellulose acetate in the presence of a catalyst, an acetyl donor, andwater or an alcohol, and under a condition that the amount of water andthe alcohol is in the range of 0.1 to 10 mol % based on the amount ofthe acetyl donor.

Each of T. R. Floyd (J. Chromatogr., 629, 243 (1993) and Kawai (Articlesof polymer (written in Japanese), Vol. 54, No. 9, 526 (1997) reports anintermolecular degree of acetyl substitution about cellulose diacetatehaving a low degree of acetyl substitution, which was evaluated by usinga reverse phase HPLC. However, there is no report about cellulosetriacetate having a high degree of acetyl substitution.

The inventors have further studied cellulose acetate having a uniformintermolecular degree of acetyl substitution, and found that celluloseacetate having the uniform degree shows an excellent solubility eventhough the substitution degree is relatively high (2.636 to 2.958). Now,a cellulose acetate product having excellent physical characteristicscan be prepared by using a solution of cellulose acetate havingexcellent solubility according to the present invention.

According to further study of the inventors, the degrees of acetylsubstitution at 2-, 3- and 6-positions can be easily and properlycontrolled by ripening cellulose acetate under a condition that theamount of water and an alcohol is in the range of 0.1 to 10 mol % basedon the amount of the acetyl donor.

The degrees of acetyl substitution at 2-, 3- and 6-positions can beeffectively controlled as is mentioned above to prepare celluloseacetate satisfying the formulas (I) to (III) or (III) to (V).

The cellulose acetate having the properly controlled degrees of acetylsubstitution makes it possible to prepare easily a cellulose acetatesolution whose solubility and viscosity can be easily controlled.Further, the invention also makes it possible to prepare easily acellulose acetate film having excellent physical and opticalcharacteristics.

The study of the inventors furthermore revealed that cellulose acetatehaving good solubility shows a specific infrared absorption spectrum inwhich an absorption maximum having a half width of 135 cm⁻¹ or less isin the range of 3450 to 3550 cm⁻¹. The absorption maximum is attributedto cellulose acetate from which a cellulose acetate solution dissolvingcellulose acetate well can be prepared. Further, from the thus-preparedsolution a cellulose acetate film having preferable properties andoptical characters can be formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a distribution curve of an intermolecularsubstitution degree of cellulose acetate prepared in Example 4.

FIG. 2 is a graph showing a distribution curve of an intermolecularsubstitution degree of cellulose acetate prepared in Comparison Example4.

FIG. 3 is a graph showing degrees of acetyl substitution regulated bythe formulas (I) to (III) and (VI) to (VIII), and also shows the degreesof acetyl substitution of cellulose acetates in Examples 1 to 9, 12 to14 and Comparison Example 1.

FIG. 4 is a graph showing degrees of acetyl substitution regulated bythe formulas (III), (IV), (VII), (VIII) and (X), and also shows thedegrees of acetyl substitution of cellulose acetates in Examples 1 to 9,12 to 14 and Comparison Examples 1 and 2.

FIG. 5 is a spectrum showing a half-width of the absorption maximum inan infrared absorption spectrum.

FIG. 6 shows infrared absorption spectra of Examples 10 and 11 andComparison Examples 5 and 6.

DETAILED DESCRIPTION OF THE INVENTION

(Preparation of Cellulose Acetate)

Cellulose as the starting material for preparation of cellulose acetatecan be obtained from cotton linters or wood pulp. A mixture of raw pulpcan be also used.

Cellulose (the starting material) is reacted in a solvent with aceticacid or acetic anhydride in the presence of a catalyst to synthesizecellulose acetate. In the synthesis, acetic acid as the solvent,sulfuric acid as the catalyst and acetic anhydride as the acetyl donorare usually used.

Migita et al. (Mokuzai Kagaku (Wood Chemistry, written in Japanese),1968, Kyoritsu Shuppan, pp. 180 to 190) describe the principle ofcellulose acetate synthesis. A typical synthesis method is a homogeneousacetylation process with a system of acetic anhydride (acetyldonor)-acetic acid (solvent)-sulfuric acid (catalyst). The processcomprises the steps of: pretreating cellulose (starting material) suchas wood pulp with an adequate amount of acetic acid, and then pouringthe material into a cooled acetylation mixture to prepare celluloseacetate. The acetylation mixture generally comprises acetic acid as thesolvent, acetic anhydride as the acetyl donor (esterifying agent) andsulfuric acid as the catalyst. The amount of acetic anhydride is usuallyin stoichiometrical excess of the total amount of water and cellulose toreact in the system. Accordingly, after the acetylation reaction iscompleted, an aqueous solution of neutralizing agent (e.g., carbonates,acetates and oxides of sodium, potassium, calcium, magnesium, iron,aluminum, zinc and ammonium) is added so as to neutralize the remainingexcess acetic anhydride and esterifying catalyst.

In a conventional process, the prepared cellulose acetate is kept at 50to 90° C. in the presence of a little amount of acetylation catalyst(generally, sulfuric acid) to conduct saponification (ripening). Thedegrees of acetyl substitution and polymerization of cellulose acetateis appropriately adjusted at the ripening process. After the celluloseacetate is ripened, the above-described neutralizing agent can be addedto neutralize completely the remaining catalyst. The cellulose acetatecan also be used at the next step without neutralizing process. Theobtained cellulose acetate solution is poured into water or dilutedacetic acid (or water or diluted acetic acid is poured into thesolution) to separate cellulose acetate from the solution. The separatedcellulose acetate is then washed, and is subjected to a stabilizingtreatment.

In Japanese Patent Provisional Publication No. 11(1999)-5851, theacetylation reaction is performed with a small amount of sulfuric acidto obtain cellulose acetate whose degree of acetyl substitution at6-position is relatively high. However, cellulose acetate prepared withsuch a small amount of sulfuric acid often has poor solubility. Further,white insoluble materials are often formed in a solution of thecellulose acetate. The acetylation reaction slowly proceeds as solidcellulose (starting material) is gradually acetylated and dissolved.Therefore, cellulose dissolved earlier is different in a reaction speedfrom cellulose dissolved later in the reaction under a condition of asmall amount of sulfuric acid. Consequently, the reaction givescellulose acetate inhomogeneous with respect to the intermoleculardegree of acetyl substitution.

The inventors have found that cellulose acetate having a uniformintermolecular degree of acetyl substitution can be prepared by ripeningcellulose acetate in the presence of a catalyst, an acetyl donor, andwater or an alcohol, and under a condition that the amount of water andan alcohol is in the range of 0.1 to 10 mol % (0.1 mol % or more andless than 10 mol %) based on the amount of the acetyl donor.

The inventors have also found that the degrees of acetyl substitution at2-, 3- and 6-positions can be easily and properly controlled by ripeningcellulose acetate in the presence of a catalyst, an acetyl donor, andwater or an alcohol, and under a condition that the amount of water andan alcohol is 0.1 to 10 mol % (0.1 mol % or more and less than 10 mol %)based on the amount of the acetyl donor.

In the case where synthesis and ripening (controlling degree of acetylsubstitution) of cellulose acetate are successively conducted,neutralizing treatment (neutralization of an acid catalyst used in thesynthesis reaction) need not be conducted, or can partially be conductedto use all or a part of the remaining catalyst in an ripening reactionat the next stage.

The acetyl donor is a compound having acetyl group (—COCH₃) that can begiven to remaining hydroxyl (—OH) of cellulose acetate through theester-exchanging reaction or esterifying reaction in the presence of acatalyst. The acetyl donor preferably is acetic acid or an acetic ester,and more preferably is acetic acid or an acetic ester of an alcohol.

According to the study of the inventors, if the amount of water and analcohol is 10 mol % or more based on the amount of the acetyl donor, theacetyl groups are liable to be eliminated from highly substitutedcellulose acetate (degrees of acetyl substitution at 2-, 3- and6-positions in total is 2.6367 or more, particularly 2.70 or more).

In contrast, when the amount of water and an alcohol is less than 10 mol% (preferably, less than 7 mol %), the acetylation of dissociatedhydroxyl proceeds at a speed comparable to the reaction speed ofelimination of acetyl groups. As a result, the reactions converge on anequilibrium point. Therefore, the amount of water and an alcohol is setto be less than 10 mol % based on the amount of the acetyl donor, andthereby the reaction between the acetyl donor and cellulose acetate ismade to be reversible. An equilibrium between a glucose unit havingremaining hydroxyl (at 2-, 3- or 6-position) with an acetyl donor(R—O—COCH₃ in which R is hydrogen or an alkyl group) and a glucose unithaving no hydroxyl (acetyl groups are combined to 2-, 3- and6-positions) with water or an alcohol (R—OH, in which R is hydrogen oran alkyl group) is controlled to obtain a uniform intermolecular degreeof acetyl substitution.

Water or an alcohol (0.1 mol % or more based on the acetyl donor) isessential for the above-mentioned equilibrium.

Further, the intramolecular degrees of substitution at 2-, 3- or6-positions can also be easily adjusted where the reaction between theacetyl donor and cellulose acetate is reversible. The equilibrium shownbelow between the acetyl donor (R—O—COCH₃ in which R is hydrogen or analkyl group) and a glucose unit having hydroxyl at 2-, 3- or 6-positionis so controlled that the degrees of acetyl substitution at 2-, 3- and6-positions can effectively be adjusted.

The catalyst used in the ripening step is preferably an acid or a metal(e.g., titanium, tin) ion. The acid may be a Lewis acid as well as anormal proton acid. If the ripening step is carried out in ethyl acetate(acetyl donor)-alcohol (solvent) system, metal alkoxides and organicbases (e.g., dialkylaminopyridine, N-methylimidazole) may be used as thecatalyst. The most preferred catalyst is an acid.

The acid catalyst preferably is a strong acid (e.g., sulfonic acid,perchloric acid, sulfuric acid, boron trifluoride, tetrafluoroboricacid). In consideration of availability, stability, toxicity andcorrosiveness, sulfuric acid is most preferred. If the reactiontemperature is 100° C. or more, acetic acid used as the acetyl donor canfunction as the acid catalyst. The present invention includes theembodiment using acetic acid as the acid catalyst as well as the acetyldonor.

The amount of catalyst is determined according to the reactiontemperature and the catalytic ability (acidity in the case of acidiccatalyst).

The ripening step has usually been carried out in acetic acid (acetyldonor)-water (solvent) system. In a conventional ripening step, however,the amount of water is 10 mol % or more based on the amount of aceticacid. Under that condition, the decomposition of ester linkage incellulose acetate predominantly proceeds, and consequently the degreesof substitution are lowered. Therefore, it is impossible to controlproperly the degrees of acetyl substitution at 2-, 3- and 6-positionsunder that conventional condition.

In the present invention, the amount of water and an alcohol is set tobe less than 10 mol % (preferably, less than 7 mol %) based on theamount of acetyl donor, and thereby the degrees of acetyl substitutionat 2-, 3- and 6positions can be properly controlled.

Generally, an excess amount of acetic anhydride is used in the step forpreparing cellulose acetate. The remaining acetic anhydride ispreferably hydrolyzed into acetic acid before the ripening step, so thatthe ripening can be carried out without acetic anhydride.

The acetyl donor can function as a solvent at the ripening step.Therefore, it is not necessary to use another solvent. However, a liquidinactive to the reaction can be used as the solvent. The amount of thesolvent other than the acetyl donor is preferably not more than fivetimes the amount of the acetyl donor. The solvent other than the acetyldonor is selected from liquid dissolving cellulose acetate at theripening step. Examples of the solvents include hydrocarbon halides(e.g., dichloromethane, chloroform, 1,1,2,2-tetrachloroethane), nitrocompounds (e.g., nitromethane, nitrobenzene), sulfones (e.g., sulfolane)and ethers (e.g., dioxane).

In the case that an acid catalyst (e.g., sulfonic acid, perchloric acid,sulfuric acid) is used, the amount of the acid catalyst at the ripeningstep is preferably in the range of 0.1 to 10 mol %, and more preferablyin the range of 0.2 to 2 mol % based on the amount of the acetyl donor.At the ripening step, the amount of the catalyst is essentially notchanged within the above-mentioned preferred range.

The ripening temperature is preferably in the range of 20 to 125° C.,and more preferably in the range of 30 to 70° C.

The ripening time is preferably in the range of 10 minutes to 10 hours,and more preferably in the range of 30 minutes to 3 hours.

The amount of water and alcohol (X) based on the acetyl donor, theamount of the catalyst (Y) based on the acetyl donor, the ripeningtemperature (T) and the ripening time (t) are preferably adjusted asimportant reaction conditions. The relation among the conditions is alsopreferably adjusted to obtain an appropriate equilibrium between theacetyl donor and cellulose acetate. For the purpose of that, theripening condition are preferably so controlled that the reactionparameter (R) defined by the following formula is more than 20. Thereaction parameter (R) is more preferably more than 30, and mostpreferably more than 40.R (Reaction parameter)=∫YZ/Xdt

In the formula, X is the molar ratio (%) of water and an alcohol basedon the amount of the acetyl donor, Y is the molar ratio (%) of thecatalyst based on the amount of the acetyl donor, Z is the temperaturereduction coefficient (3^((T−30)/20) in which T is the reactiontemperature (° C.)), and t is the reaction time (unit: minute). In thecase where X is less than 0.1 mol %, X is regarded as 0.1 to calculatethe R.

(Intermolecular Substitution Degree of Cellulose Acetate)

The process according to the present invention can form celluloseacetate in which an intermolecular acetyl substitution is uniform.

Cellulose acetate having a uniform degree of acetyl substitution can bepurified (for example by a chromatography) from cellulose acetate havingdifferent degrees of acetyl substitution. However, cellulose acetatehaving a uniform degree of acetyl substitution is preferably obtained ina more economical way, in which cellulose acetate can be directlysynthesized by improving the process as is mentioned above.

The average degree of acetyl substitution is preferably in the range of2.636 to 2.958.

The degree of acetyl substitution can be measured by NMR according to amethod of Tezuka (Carbohydr. Res., 273, 83 (1995)). Free hydroxyl groupsin a cellulose acetate sample are reacted with propionic anhydride toform propionate ester in pyridine. The obtained sample is dissolved inchloroform-d₁, and the spectrum of carbon-13 is measured. The acetylcarbon signals are shown in the range of 169 ppm to 171 ppm in the orderof 2-, 3- and 6-positions from a higher magnetic field. The propionylcarbon signals are shown in the range of 172 ppm to 174 ppm in the sameorder. The ratio of acetyl to propionyl at the corresponding positioncan give the degree of acetylation in the original cellulose acetate.

The average degree of acetyl substitution of cellulose acetate hasconventionally determined from a value of acetic acid content, which canbe measured according to ASTMD-817-91 (testing method for celluloseacetate or the like). The acetic acid content obtained according to ASTMcan be converted into the degree of substitution according to thefollowing formula.DS=162×AV×0.01/(60−40×AV×0.001)

In the formula, DS is the degree of acetyl substitution, and AV is theacetic acid content (%).

The calculated degree of substitution may usually be somewhat differentfrom the value measured by NMR described above. If the values aredifferent from each other, the value measured by NMR is prior to theASTM value. If the values measured by various NMR methods are differentfrom each other, the value measured according to the method of Tezuka isprior to the other values.

In the present specification, the uniform intermolecular degree ofacetyl substitution means that cellulose acetate shows a distributioncurve of an intermolecular substitution degree in which the maximum peakhas a half width of less than 0.080, or has a half width of less than Ydefined in the following formula:Y=−0.17788X+0.5788in which X is the degree of acetyl substitution.

The cellulose acetate most preferably shows a distribution curve of anintermolecular substitution degree in which the maximum peak has a halfwidth of less than 0.080 and less than Y defined in the formula.

The distribution curve of an intermolecular substitution degree ofcellulose acetate can be obtained by converting the abscissa (thehorizontal axis) of an elution curve (elution time) to the degree ofacetyl substitution (0 to 3). The elution curve is measured by a reversephase HPLC.

The conditions of the reverse phase HPLC are shown below.

Eluent: Linear gradient by 28 minutes from chloroform/methanol (9/1,v/v)):methanol/water (8/1, v/v) =20: 80 to chloroform/methanol (9/1,v/v)=100

Column: Nova Packphenyl of 3.9 × 150 mm (Waters) Column temperature: 30°C. Flow rate: 0.7 ml per minute Conc. of sample: 2 mg per ml Injectedamount: 20 μl Detector: Evaporative light scattering detector(ELSD-MK-III, Varex) Drift tube temp.: 80° C. Gas flow: 2.1 SLPM

Before the elution curve is converted into the distribution curve of anintermolecular substitution degree, at least four samples havingdifferent substitution degrees are measured under the same conditions todetermine elution time. A formula of converting the elution time (T) tothe degree of substitution (DS) can be obtained from the determinedelution time. The relation between the elution time (T) and the degreeof substitution (DS) can give a calibration curve according to the leastsquares method. The function is usually given in a secondary formulashown below.DS=aT ² +bT+c

In the formula, DS is the degree of acetyl substitution, T is theelution time, and a, b and c are coefficients of the conversion formula.

FIG. 1 is a graph showing a distribution curve of an intermolecularsubstitution degree of cellulose acetate prepared in Example 4.

FIG. 2 is a graph showing a distribution curve of an intermolecularsubstitution degree of cellulose acetate prepared in Comparison Example4.

The DS of the abscissa (the horizontal axis) in FIGS. 1 & 2 means thedegree of acetyl substitution. The intensity of the ordinate (thevertical axis) in FIGS. 1 & 2 means the amount of cellulose acetatehaving the degree of acetyl substitution corresponding to the abscissa.

The distribution curve of an intermolecular substitution in FIG. 1 showsthe maximum peak (E) at the degree of substitution of 2.795. Thedistribution curve of an intermolecular substitution in FIG. 2 shows themaximum peak (E) at the degree of substitution of 2.851.

A base line (A-B) tangent is drawn from the base point (A) on the lowsubstitution degree side to the base point (B) on the high substitutiondegree side. Independently, a line perpendicular to the horizontal axisis drawn from the maximum peak (E) of the curve to determine theintersection (C) of the perpendicular line and the base line (A-B). Themidpoint (D) between the peak (E) and the intersection (C) is thendetermined. A line including the midpoint (D) is drawn parallel to thebase line (A-B) to determine two intersections (A′, B′) of the line andthe distribution curve of the intermolecular substitution. From each ofthe intersections (A′, B′), a line perpendicular to the horizontal axisis drawn. The interval between the feet of the thus-drawn perpendicularsis defined as the half-width of the maximum peak.

(Intramolecular Substitution Degree of Cellulose Acetate)

The process according to the present invention can also form celluloseacetate in which degrees of acetyl substitution at 2-, 3- and6-positions are properly controlled.

A cellulose acetate film having preferable properties and opticalcharacters can be made of cellulose acetate in which the degrees ofacetyl substitution at 2-, 3- and 6- positions satisfy the followingformulas (I) to (III).2DS+3DS<6DS×4−1.70  (I)2DS+3DS<−6DS×4+5.70  (II)2DS+3DS>1.80  (III)

In the formulas (I) to (III), 2DS is the degree of acetyl substitutionat 2-position, 3DS is the degree of acetyl substitution at 3-position,and 6DS is the degree of acetyl substitution at 6-position.

The degrees of acetyl substitution at 2-, 3- and 6-positions alsopreferably satisfy the following formula (VI).2DS+3DS−6DS<1  (VI)

In the formula (VI), 2DS is the degree of acetyl substitution at2-position, 3DS is the degree of acetyl substitution at 3-position, and6DS is the degree of acetyl substitution at 6-position.

The degrees of acetyl substitution at 2- and 3- positions furtherpreferably satisfy the following formula (VII), and furthermorepreferably satisfy the following formula (VIII).2DS+3DS>1.82  (VII)2DS+3DS>1.84  (VIII)

FIG. 3 is a graph showing the conditions of degrees of acetylsubstitution regulated by the formulas (I) to (III), (VI) to (VIII), andthe graph also shows the degrees of acetyl substitution of celluloseacetates in Examples 1 to 9, 12 to 14 and Comparison Example 1. In thegraph, the sum of the degrees of acetyl substitution at 2- and3-positions (2DS and 3DS) are plotted on the abscissa (the horizontalaxis), and the degree of acetyl substitution at 6-position (6DS) isplotted on the ordinate (the vertical axis).

Cellulose acetate satisfying the formula (I) is shown in the area belowthe line of (I) in FIG. 3 in which 2DS+3DS=6DS×4−1.70.

Cellulose acetate satisfying the formula (II) is shown in the area abovethe line of (II) in FIG. 3 in which 2DS+3DS=−6DS×4+5.70.

Cellulose acetate satisfying the formula (III) is shown in the area atthe right side of the line of (III) in FIG. 3 in which 2DS+3DS=1.80.

Cellulose acetate satisfying the formula (VI) is shown in the area atthe left side of the line of (VI) in FIG. 3 in which 2DS+3DS−6DS=1.

Cellulose acetate satisfying the formula (VII) is shown in the area atthe right side of the line of (VII) in FIG. 3 in which 2DS+3DS=1.82.

Cellulose acetate satisfying the formula (VIII) is shown in the area atthe right side of the line of (VIII) in FIG. 3 in which 2DS+3DS=1.84.

The solid circles 1 to 9 and 12 to 14 correspond to Examples 1 to 9 and12 to 14, respectively. The open circle C1 corresponds to ComparisonExample 1. Cellulose acetate of Comparison Example 2 is out of the rangeshown in the graph.

A cellulose acetate solution whose solubility and viscosity are easilycontrolled can be prepared from the cellulose acetate in which thedegrees of acetyl substitution at 2-, 3- and 6-positions satisfy thefollowing formulas (III) to (V):2DS+3DS>1.80  (III)3DS<2DS  (IV)6DS>0.80  (V)

In the formulas (III) to (V), 2DS is the degree of acetyl substitutionat 2-position, 3DS is the degree of acetyl substitution at 3-position,and 6DS is the degree of acetyl substitution at 6-position.

The degrees of acetyl substitution at 2- and 3- positions furtherpreferably satisfy the following formula (VII), and furthermorepreferably satisfy the following formula (VIII).2DS+3DS>1.82  (VII)2DS+3DS>1.84  (VIII)

The degrees of acetyl substitution at 2- and 3- positions alsopreferably satisfy the following formula (X).3DS>2DS×2−1  (V)

FIG. 4 is a graph showing the conditions of degrees of acetylsubstitution regulated by the formulas (III), (IV), (VII), (VIII) and(X), and also shows the degrees of acetyl substitution of celluloseacetates in Examples 1 to 9, 12 to 14 and Comparison Examples 1 and 2.In the graph, the degree of acetyl substitution at 2-positions (2DS) isplotted on the abscissa (the horizontal axis), and the degree of acetylsubstitution at 3-position (3DS) is plotted on the ordinate (thevertical axis).

Cellulose acetate satisfying the formula (III) is shown in the areaabove the line of (III) in FIG. 4 in which 2DS+3DS=1.80.

Cellulose acetate satisfying the formula (IV) is shown in the area belowthe line of (IV) in FIG. 4 in which 3DS=2DS.

Cellulose acetate satisfying the formula (VII) is shown in the areaabove the line of (VII) in FIG. 4 in which 2DS+3DS=1.82.

Cellulose acetate satisfying the formula (VIII) is shown in the areaabove the line of (VIII) in FIG. 4 in which 2DS+3DS=1.84.

Cellulose acetate satisfying the formula (X) is shown in the area at theleft side of the line of (X) in FIG. 4 in which 3DS=2DS×2−1.

The solid circles 1 to 5 and 7 to 10 correspond to Examples 1 to 5 and 7to 10, respectively. The open circles C1 and C2 correspond to ComparisonExamples 1 and 2, respectively.

Cellulose acetate preferably satisfies at least four formulas, morepreferably satisfies at least five formulas, further preferablysatisfies at least six formulas, furthermore preferably satisfies atleast seven formulas, and most preferably satisfies at least eightformulas of (I) to (X). Cellulose acetate particularly preferablysatisfies all the formulas (I) to (X).

The degrees of acetyl substitution at 2-, 3- and 6-positions can bedetermined by means of ¹³C-NMR after the cellulose acetate is subjectedto a process of forming propionate ester. The measurement of degrees ofacetyl substitution is described in detail in Tezuka et al. (Carbohydr.Res., 273, 83-91 (1995)).

(Infrared Absorption Spectrum)

The process according to the present invention can also form celluloseacetate, which shows an infrared absorption spectrum in which anabsorption maximum having a half width of 135 cm⁻¹ or less is in thewave number range of 3450 to 3550 cm⁻¹. The absorption maximum is morepreferably in the range of 3455 to 3540 cm⁻¹, and further preferably inthe range of 3460 to 3530 cm⁻¹. The half width of the absorption maximumis more preferably 130 cm⁻¹ or less, and further preferably 125 cm⁻¹ orless.

The infrared absorption spectrum of cellulose acetate is measured in theform of a film, which is formed by the solvent cast method. Concreteprocedures of the measurement are described in Example 6.

From the measured spectrum (ordinate: absorbance), the position and thehalf-width of the absorption maximum are obtained. The analysis of theinfrared absorption spectrum is described in “Kobunshi no Kozo(Structure of polymer), written in Japanese”, H. Tadokoro, (1976),219-221, KagakuDojin.

FIG. 5 is a spectrum showing a half-width of the absorption maximum inan infrared absorption spectrum.

In the spectrum in FIG. 5, there is an absorption band attributed tohydroxyl at about 3500 cm⁻¹. The half-width is obtained in the followingmanner. First, a base line (A-B) tangent to both of the base point (A)on the higher energy side (at about 3700 cm⁻¹) and the base point (B) onthe lower energy side (at about 3250 cm⁻¹) is drawn. Independently, aline perpendicular to the horizontal axis is drawn from the peak (E) ofthe band so as to determine the intersection (C) of the perpendicularand the base line (A-B). The midpoint (D) between the peak (E) and theintersection (C) is then determined, and a line including the midpoint(D) is drawn parallel to the base line (A-B) to determine intersections(A′, B′) of the line and the spectrum. From each of the intersections(A′, B′), a line perpendicular to the horizontal axis is drawn. Theinterval between the feet of the thus-drawn perpendiculars is defined asthe half-width (Δν_(1/2)).

If the absorption band has two or more peaks in the range of 3459 to3550 cm⁻¹, the maximum peak is regarded as the peak of the band (E inFIG. 5).

The infrared absorption spectrum of methylcellulose is described in“Cellulose”, 4, 281(1997), Kondo et al.

It is also preferred for an already produced cellulose acetate film(such as a film product)) to show an infrared absorption spectrum inwhich an absorption maximum having a half width of 135 cm⁻¹ or less isin the range of 3450 to 3550 cm⁻¹. In that case, since additives (forexample, ultraviolet absorbers) often affect the spectrum, theabsorption spectrum attributed to cellulose acetate itself isselectively measured.

(Organic Solvent)

Cellulose acetate is usually dissolved in an organic solvent to preparea cellulose acetate solution, from which various products (e.g., film)are produced. Examples of the organic solvent include ketones, esters,ethers, hydrocarbons and alcohols.

Although halogenated hydrocarbons such as methylene chloride are alsousable from the technical viewpoint, they are not preferred inconsideration of the environment. The organic solvent, therefore,preferably contains halogenated hydrocarbons in an amount of less than 5wt. % (more preferably less than 2 wt. %). It is also preferred that nohalogenated hydrocarbon be found in the produced cellulose acetate film.

The organic solvent preferably contains a solvent selected from thegroup consisting of an ether having 2 to 12 carbon atoms, a ketonehaving 3 to 12 carbon atoms and an ester having 2 to 12 carbon atoms.

The ether, the ketone or the ester may have a cyclic structure. Acompound having two or more functional groups of ether, ketone or ester(—O—, —CO— or —COO—) is also usable as the solvent. The organic solventmay have other functional groups such as alcoholic hydroxyl. If thesolvent is the compound having two or more functional groups, the numberof carbon atoms is in any of the above ranges.

Examples of the ether having 2 to 12 carbon atoms include dimethylether, methyl ethyl ether, diisopropyl ether, dimethoxymethane,dimethoxyethane, 1,4-dioxane, 1,3- dioxolan, tetrahydrofuran, anisoleand phenetole.

Examples of the ketone having 3 to 12 carbon atoms include acetone,methyl ethyl ketone, diethyl ketone, diisobutyl ketone, cyclohexanone,methylcyclohexane and acetylacetone.

Examples of the ester having 2 to 12 carbon atoms include methylformate, ethyl formate, propyl formate, pentyl formate, methyl acetate,ethyl acetate and pentyl acetate. An organic solvent containing methylacetate in an amount of 50 wt. % or more is particularly preferablyused.

Examples of the compound having two or more functional groups include2-ethoxyethyl acetate, 2-methoxy ethanol and 2-butoxy ethanol.

A particularly preferred solvent is a mixture of three different kindsof solvents. In the mixture, the first solvent is a ketone having 3 to12 carbon atoms or an ester having 2 to 12 carbon atoms, the secondsolvent is a monovalent straight-chained alcohol having 1 to 5 carbonatoms, and the third solvent is an alcohol having a boiling point of 30to 170° C. or a hydrocarbon having a boiling point of 30 to 170° C.

The ketone or ester used as the first solvent is the same as thatdescribed above. The first solvent may be a mixture thereof. Forexample, a mixture of ketone (e.g., acetone) and ester (e.g., methylacetate) can be used as the first solvent.

The second solvent is a monovalent straight-chained alcohol having 1 to5 carbon atoms. In the alcohol, hydroxyl may be connected to either theterminal of the straight hydrocarbon chain (i.e., primary alcohol) orthe middle of the chain (i.e., secondary alcohol). Examples of thealcohol for the second solvent include methanol, ethanol, 1-propanol,2-propanol, 1-butanol, 2-butanol, 1-pentanol, 2-pentanol and 3-pentanol.The number of carbon atoms in the alcohol is preferably 1 to 4, morepreferably 1 to 3, and most preferably 1 or 2. Ethanol is particularlypreferred.

The third solvent is an alcohol having a boiling point of 30 to 170° C.or a hydrocarbon having a boiling point of 30 to 170° C. The alcohol ispreferably monovalent. The hydrocarbon moiety of the alcohol may have astraight chain structure, a branched chain structure or a cyclicstructure. The hydrocarbon moiety is preferably a saturated aliphatichydrocarbon. The alcohol for the third solvent may be a primary alcohol,a secondary alcohol or a tertiary alcohol.

Examples of the alcohol for the third solvent include methanol (boilingpoint: 64.65° C.), ethanol (boiling point: 78.325° C.), 1-propanol(boiling point: 97.15° C.), 2-propanol (boiling point: 82.4° C.),1-butanol (boiling point: 117.9° C.), 2-butanol (boiling point: 99.5°C.), t-butyl alcohol (boiling point: 82.45° C.), 1-pentanol (boilingpoint: 137.5° C.), 2-methyl-2-butanol (boiling point: 101.9° C.),cyclohexanol (boiling point: 161° C.), 2-fluoroethanol (boiling point:103° C.), 2,2,2-trifluoroethanol (boiling point: 80° C.),2,2,3,3-tetrafluoro-1-propanol (boiling point: 109° C.),1,3-difluoro-2-propanol (boiling point: 55° C.),1,1,1,3,3,3-hexafluoro-2-methyl-2-propanol (boiling point: 62° C.),1,1,1,3,3,3-hexafluoro-2-propanol (boiling point: 59° C.),2,2,3,3,3-pentafluoro-1-propanol (boiling point: 80° C.),2,2,3,4,4,4-hexafluoro-1-butanol (boiling point: 114° C.),2,2,3,3,4,4,4-heptafluoro-1-butanol (boiling point: 97° C.),perfluoro-tert-butyl alcohol (boiling point: 45° C.),2,2,3,3,4,4,5,5-octafluoro-1-pentanol (boiling point: 142° C.),2,2,3,3,4,4-hexafluoro-1,5-pentanediol (boiling point: 111.5° C.),3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1octanol (boiling point: 95°C.), 2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluoro-1-octanol (boilingpoint: 165° C.), 1-(pentafluorophenyl)ethanol (boiling point: 82° C.)and 2,3,4,5,6-pentafluorobenzyl alcohol (boiling point: 115° C.).

The alcohol for the third solvent is defined as the same as that for thesecond solvent described above. However, the alcohol for the thirdsolvent is selected so that it may be different from the alcohol for thesecond solvent. For example, if ethanol is used as the second solvent,other alcohols defined in the description for the second solvent (e.g.,methanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 1-pentanol,2-pentanol and 3-pentanol) can be used as the third solvent.

The hydrocarbon for the third solvent may have a straight chained,branched chained or cyclic structure. The hydrocarbon may be eitheraliphatic or aromatic. If it is aliphatic, the hydrocarbon may be eithersaturated or unsaturated.

Examples of the hydrocarbon include cyclohexane (boiling point: 80.7°C.), hexane (boiling point: 69° C.), benzene (boiling point: 80.1° C.),toluene (boiling point: 110.6° C.) and xylene (boiling point: 138.4 to144.4° C.).

In the mixed solvent of three different kinds of solvents, the contentof the first solvent is preferably in the range of 50 to 95 wt. %, morepreferably in the range of 60 to 92 wt. %, further preferably in therange of 65 to 90 wt. %, and most preferably in the range of 70 to 88wt. %. The content of the second solvent is preferably in the range of 1to 30 wt. %, more preferably in the range of 2 to 27 wt. %, furtherpreferably in the range of 3 to 24 wt. %, and most preferably in therange of 4 to 22 wt. %. The content of the third solvent is preferablyin the range of 1 to 30 wt. %, more preferably in the range of 2 to 27wt. %, further preferably in the range of 3 to 24 wt. %, and mostpreferably in the range of 4 to 22 wt. %.

To the mixed solvent, another organic solvent is added so as to preparea mixed solvent of four solvents. In that case, the fourth solvent ispreferably selected from the three kinds of solvents described above.Further, ethers having 3 to 12 carbon atoms (e.g., diisopropyl ether,dimethoxyethane, diethoxyethane, 1,4-dioxane, 1,3-dioxofuran,tetrahydrofuran, anisole, phenetole) and nitromethane can be usedbesides the above three kinds of solvents.

The organic solvent has a boiling point preferably in the range of 20 to300° C., more preferably in the range of 30 to 200° C., furtherpreferably in the range of 40 to 100° C., and most preferably in therange of 50 to 80° C.

Into the above organic solvent, cellulose acetate is dissolvedpreferably by the cooling dissolution method. The cooling dissolutionmethod comprises a swelling step, a cooling step and a warming step.Even if cellulose acetate can be dissolved in a solvent at roomtemperature, the cooling dissolution method makes it possible to preparea homogeneous solution rapidly.

(Swelling Step)

At the first step, cellulose acetate is mixed with a solvent to swellthe polymer in the solvent. The swelling step is preferably conducted ata temperature of −10 to 55° C. The swelling step is usually conducted atroom temperature.

The ratio of cellulose acetate to the mixture is determined depending ona concentration of a solution to be obtained. The amount of celluloseacetate in the mixture is preferably in the range of 5 to 30 wt. %, morepreferably in the range of 8 to 20 wt. %, and most preferably in therange of 10 to 15 wt. %.

The mixture of cellulose acetate and the solvent is preferably stirreduntil the cellulose acetate is enough swelled. The stirring time ispreferably in the range of 10 to 150 minutes, and more preferably in therange of 20 to 120 minutes.

At the swelling step, additives such as a plasticizer, a deteriorationinhibitor, a dye and an ultraviolet absorbent can be added to themixture.

(Cooling Step)

At the next step, the swelled mixture is cooled to a temperature of −100to −10° C. The swelled mixture preferably solidifies at the coolingstep.

The cooling rate is preferably 4° C. per minute or more, more preferably8° C. per minute or more, and most preferably 12° C. per minute or more.The cooling rate is preferably as fast as possible. However, atheoretical upper limit of the cooling rate is 10,000° C. per second, atechnical upper limit is 1,000° C. per second, and a practical upperlimit is 100° C. per second.

The cooling rate means the change of temperature at the cooling step perthe time taken to complete the cooling step. The change of temperaturemeans the difference between the temperatures at which the cooling stepis started and at which the cooling step is completed. The cooling ratein the examples of Japanese patent Provisional publication Nos.9(1997)-95544, 9(1997)-95557 and 9(1997)-95538 is about 3° C. perminute.

The mixture is preferably cooled in a sealed vessel to preventcontamination of water, which may be caused by dew condensation at thecooling step. Further, the mixture may be cooled under a reducedpressure, and thereby the time taken to complete the cooling step can beshortened. A vessel resisting pressure is preferably used to conduct theprocedures under a reduced pressure.

Various methods and apparatus can be used to perform the cooling step.

For example, the swelled mixture is conveyed while stirred into acylinder, which is then cooled from the outside, and thereby the mixtureis rapidly and homogeneously cooled. For this procedure, an apparatuscomprising a cylindrical vessel, a screw conveyer which transports themixture while stirring into the vessel and a cooling means providedaround the vessel is preferably used.

A supplemental solvent beforehand cooled at a temperature of −105 to−15° C. may be added to the swelled mixture, so as to cool more quickly.

Further, the swelled mixture can be cooled further quickly by extrudingthe mixture into a liquid beforehand cooled at a temperature of −100 to−10° C. The extruded mixture is in the form of fiber having a diameterin the range of 0.1 to 20.0 mm. There is no specific limitation withrespect to the liquid for cooling the mixture.

If the swelled mixture is extruded into the cooled liquid, the extrudedmixture in the form of fiber is preferably separated from the cooledliquid after the cooling step and before the warming step.

The extruded mixture usually solidifies to gel at the cooling step, andhence it is easy to separate the solid fiber from a liquid. For example,the solid fiber in the liquid can be taken out in a net. A board havingsmall holes or slits can be used in place of the net. The net or theboard is made of a plastic or metal that is not dissolved in the cooledliquid. The mesh of the net, the diameter of the hole or the width ofthe slit should be adjusted to the diameter of the fiber to prevent thefiber from passing through the net or the board. Further, the conveyermay be made of a net so as to separate the fiber from the liquid whiletransporting the fiber from a cooling device to a warming device.

(Warming Step)

The cooled mixture is warmed to a temperature of 0 to 200° C. Thetemperature of the obtained solution after the warming step is usuallyroom temperature.

The warming rate is 4° C. per minute or more, more preferably 8° C. perminute or more, and most preferably 12° C. per minute or more. Thewarming rate is preferably as fast as possible. However, a theoreticalupper limit of the cooling rate is 10,000° C. per second, a technicalupper limit is 1,000° C. per second, and a practical upper limit is 100°C. per second.

The warming rate means the change of temperature at the warming step perthe time taken to complete the warming step. The change of temperaturemeans the difference between the temperature at which the warming stepis started and the temperature at which the warming step is completed.The warming rate in the examples of Japanese patent Provisionalpublication Nos. 9(1997)-95544, 9(1997)-95557 and 9(1997)-95538 is about3° C. per minute.

The mixture may be warmed under an increased pressure, and thereby thetime taken to complete the warming step can be shortened. A vesselresisting pressure is preferably used to conduct the procedures under anincreased pressure.

Various methods and apparatus can be used to perform the warming step.

For example, the swelled mixture is conveyed while stirred into acylinder, which is then warmed from the outside, and thereby the mixtureis rapidly and homogeneously warmed. For this procedure, an apparatuscomprising a cylindrical vessel, a screw conveyer which transports themixture while stirring into the vessel and a warming means providedaround the vessel is preferably used.

The swelled mixture can be warmed further quickly by extruding themixture into a liquid beforehand warmed. The extruded mixture is in theform of fiber having a diameter in the range of 0.1 to 20.0 mm. There isno specific limitation with respect to the liquid for warming themixture.

If the mixture is extruded in the form of a fiber at the cooling step,the cooled fiber is immersed in the beforehand warmed liquid at thewarming step. If the cooling step is conducted by other procedures, thecooled mixture is extruded in the form of a fiber into the warmedliquid. If the extrusion into a fiber is successively performed, theproduced cellulose acetate solution can be used as the liquid forwarming the next swelled mixture. In that case, the swelled mixture inthe form of a fiber is immersed into a warm cellulose acetate solutionproduced before, so as to warm the mixture rapidly to prepare a newcellulose acetate solution, which is then used as the liquid for warmingthe next mixture.

Further, the cooled swelled mixture may be introduced into a cylindricalvessel, in which the flow of the mixture is repeatedly divided androtated. While repeatedly divided and rotated, the mixture is warmedfrom the outside of the vessel. The vessel having a partition by whichthe flow of the mixture is divided and rotated is generally known as astatic mixer. For example, in a typical static mixer (TM, Kenix), twoelements are provided. One of them divides the mixture into two flows,and rotates the flows counterclockwise (counterclockwise element) by180°. The other divides the mixture into two flows, and rotates theflows clockwise (clockwise element) by 180°. Those elements are placedperpendicularly to each other.

The swelled mixture may be heated to a temperature above the boilingpoint of the solvent under a pressure controlled so that the solvent maynot boil. The temperature is determined according to the solvent, but isgenerally in the range of 60 to 200° C. The pressure is determined inconsideration of the temperature and the boiling point, but is generallyin the range of 1.2 to 20 kgw/cm².

(Post Treatment After Preparation of Solution)

The prepared solution can be subjected to post treatment such asadjustment of concentration (or dilution), filtration, and adjustment oftemperature or addition of components.

The additional components are determined according to use of celluloseacetate solution. Examples of the representative additives include aplasticizer, a deterioration inhibitor (e.g., a peroxide decomposer, aradical inhibitor, a metal inactivator, an acid scavenger), a dye and anultraviolet absorbent. In this step, fine particles (preferably, fineparticles dispersed in a diluted cellulose acetate solution) arepreferably added.

(Fine Particles)

The cellulose acetate film of the invention can contain fine particleshaving a mean particle size of 1.0 μm or less. The fine particlesfunction as a slipping agent, and improve the kinetic frictioncoefficient of the film.

The fine particles are preferably inorganic compounds. Examples of theinorganic compound include silicon dioxide, titanium dioxide, aluminumoxide, zirconium oxide, calcium carbonate, talc, clay, burned kaolin,burned calcium silicate, hydrated calcium silicate, aluminum silicate,magnesium silicate and calcium phosphate. Preferred are silicon dioxide,titanium dioxide and zirconium oxide, and particularly preferred issilicon dioxide.

Onto the surface of the inorganic fine particles, methyl can beintroduced through a surface treatment. For example, fine particles ofsilicon oxide are treated with dichlorodimethylsilane orbis(trimethylsilyl)amine.

Fine particles of silicon oxide are commercially available (e.g.,Aerozil R972TM, R972DTM, R974TM and R812TM, from Japan Aerozil Co.,Ltd.). Fine particles of zirconium oxide are also commercially available(e.g., Aerozil R976TM and R811TM, from Japan Aerozil Co., Ltd.).

The mean particle size of the fine particles is preferably 1.0 μm orless, more preferably in the range of 0.1 to 1.0 μm, and most preferablyin the range of 0.1 to 0.5 μm.

The amount of the fine particles is preferably in the range of 0.005 to0.3 wt. %, more preferably in the range of 0.01 to 0.1 wt. % based onthe amount of cellulose acetate.

The fine particles may be added in any step of the film forming processdescribed below. Preferably, a diluted solution analogous to the organicsolution of cellulose acetate is prepared, and in the diluted solutionthe fine particles are dispersed. The thus-prepared dispersion and theorganic solution of cellulose acetate are mixed, and from the mixture afilm is prepared. Thus, a film containing the fine particles evenlydispersed can be obtained.

(Plasticizer)

The cellulose acetate film generally contains a plasticizer.

As the plasticizer, phosphoric esters and carboxylic esters are used.Examples of the phosphoric esters include triphenyl phosphate, tricresylphosphate, octyldiphenyl phosphate, triethyl phosphate and tributylphosphate. Typical carboxylic esters are phthalic esters, citric esters,oleic esters and linoleic esters. Examples of the phthalic estersinclude dimethyl phthalate, diethyl phthalate, dibutyl phthalate,dimethoxy phthalate, dioctyl phthalate and diethylhexyl phthalate.Examples of the citric esters include triethyl acetylcitrate andtributyl acetylcitrate. Examples of the oleic esters include butyloleate. Examples of other carboxylic esters include ethylphthalylethylglycolate, butylphthalylbutyl glycolate, triacetin, metylacetylricinolate, dibutyl sebacate and various trimellitic esters.

The amount of plasticizer is generally in the range of 0.1 to 40 wt. %,more preferably in the range of 1.0 to 20 wt. % based on the amount ofcellulose acetate.

(Deterioration Inhibitor)

The cellulose acetate film preferably contains a deteriorationinhibitor. Examples of the deterioration inhibitor include a peroxidedecomposer, a radical inhibitor, a metal inactivator and an acidscavenger. The deterioration inhibitor is described in Japanese PatentProvisional Publication Nos. 3(1991)-199201, 5(1993)-1907073,5(1993)194789, 5(1993)-271471 and 6(1994)-107854. A particularlypreferred example of the deterioration inhibitor is butylatedhydroxytoluene (BHT).

The amount of deterioration inhibitor is preferably in the range of 0.01to 0.5 wt. %, more preferably in the range of 0.05 to 0.2 wt. % based onthe amount of cellulose acetate.

(Ultraviolet Absorbent)

The cellulose acetate film may contain an ultraviolet absorbent, whichimproves aging stability of the film. The ultraviolet absorbentpreferably has no absorption band in the visible wavelength region.

Examples of the ultraviolet absorbent include benzophenone compounds(e.g., 2,4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone,2-hydroxy-4-n-octoxybenzophenone, 4-dodecyloxy-2-hydroxy benzophenone,2,2′,4,4′-tetrahydroxybenzophenone, 2,2′-dihydroxy-4,4′dimethoxybenzophenone), pentotriazole compounds (e.g.,2-(2′-hydroxy-5-methylphenyl)benzotriazole,2-(2′-hydroxy-3′,5′-di-t-butylphenyl)benzotriazole,2-(2′-hydroxy-3′-dit-butyl-5′-methylphenyl)benzotriazole) and salicyliccompounds (e.g., phenyl salicylate, methyl salicylate).

The amount of ultraviolet absorbent preferably in the range of 0.5 to 20wt. %, more preferably in the range of 1 to 10 wt. % based on the amountof cellulose acetate film.

(Dye)

A dye may be incorporated into the cellulose acetate film, so as toprevent the light piping phenomenon.

The hue of the dye is preferably gray. A compound showing good heatresistance in the temperature range for preparing the cellulose acetatefilm and having good compatibility with cellulose acetate is preferablyused as the dye.

Two or more dyes may be used in combination.

(Film Formation)

The process for preparing the organic solution of cellulose acetate(dope) according to the cooling dissolution method is very differentfrom a usual process (in which a material mixture is only stirred atroom temperature or elevated temperature) for preparing a solution forthe solvent cast method. However, the step for forming a film from theprepared solution can be carried out in the same manner.

The cellulose acetate solution is cast on a support, and the solvent isevaporated to form a film. Before casting the solution, theconcentration of the solution is preferably so adjusted that the solidcontent of the solution is in the range of 18 to 35 wt. %. The surfaceof the support is preferably polished to give a mirror plane. A drum ora band is used as the support. The casting and drying steps of thesolvent cast method are described in U.S. Pat. Nos. 2,336,310,2,367,603, 2,492,078, 2,492,977, 2,492,978, 2,607,704, 2,739,069,2,739,070, British Patent Nos. 640,731, 736,892, Japanese PatentPublication Nos. 45(1970)-4554, 49(1974)-5614, Japanese PatentProvisional Publication Nos. 60(1985)-176834, 60(1985)203430 and62(1987)-115035.

Preferably, the cellulose acetate film formed on the support is peeledoff before completely dried (while the organic solvent still remains inan amount of 30 wt. % or more based on the weight of the film), and thenis further dried. For this procedure, the solution cast on the supportmust gel quickly. In promoting the gelation, a poor solvent such asalcohols (the above-described third solvent) is effective. The castingmethod can be modified to promote the gelation.

The solution is cast on the support beforehand cooled at 10° C. orbelow, and thereby the gelation is promoted (ref. Japanese PatentPublication No. 5(1993)-17844). The support can be cooled with coolingmedia or cold air. In this cooling step, dry air may be blown over thesupport for 2 or more seconds to dry the film on the support. The formedfilm is peeled off the support, and can be further dried with air toremove the solvent remaining in the film. The temperature of the air canbe gradually elevated from 100 to 160° C.

Otherwise, the solution is cast on the support beforehand warmed at 30°C. or more, and then the support is cooled to 20° C. or below. Throughthis procedure, the gelation can be also promoted (ref. Japanese PatentProvisional Publication Nos. 61(1986)-148013 and 61(1986)-158413). Thesupport can be warmed with a heater provided on the surface of thesupport, or hot air may be blown over the support. Further, hot watermay be circulated in the drum support to warm the film. It is preferredthat the temperature be elevated immediately after the solution is caston the support. At the beginning of warming, much latent heat isrequired because of evaporation of the solvent. Accordingly, besides theabove heating means, auxiliary heating means such as heaters (steamheater, IR heater) are preferably used to heat the support, or hot airis preferably blown over the bottom surface of the support at thebeginning of warming. For cooling the support, the warmed support may beleft to cool down. Otherwise, the support may be forced to cool down byblowing cold air or by circulating cold water.

Having excellent optical character and properties, the thus-formedcellulose acetate film can be widely used. The film of the invention isparticularly effective in liquid crystal displays.

The cellulose acetate film may be subjected to AG (anti-glare) treatmentor AR (anti-reflection) treatment. The AG treatment improves thetransmittance of the film by about 3%. In the AR treatment, ananti-reflection film (consisting of one layer, two layers or three ormore layers) is provided to lower loss by reflection. Materials for theanti-reflection film are described in ‘Thin Film Handbook (written inJapanese)’, Ohom-sha, Dec. 10 (1983), 818-821.

(Protective Film for Polarizing Plate and Liquid Crystal Display)

The cellulose acetate film is particularly preferably used in a liquidcrystal display as a protective film for polarizing plate or a phaseretarder.

A liquid crystal display generally has a liquid crystal display deviceand a polarizing plate.

The liquid crystal display device comprises a liquid crystal layer, asubstrate supporting the layer and an electrode layer which has afunction of applying a voltage to the liquid crystal. The substrate andthe electrode layer are made of transparent materials for displaying. Asthe transparent substrate, a glass thin plate or a resin film is used.If the device must have slight flexibility, a resin film must be used.In addition to high transparency, the transparent substrate must have alow birefringent index and a high heat-resistance. The device may have aphase retarder, which is a birefringent film for removing undesirablecolors of displayed images. A resin film is also used as the phaseretarder, but the phase retarder must have a high birefringent index

The polarizing plate comprises a protective film and a polarizingmembrane. The polarizing membrane is a resin film containing iodine anddichromatic dye for polarizing. The protective film is provided on oneor each surface of the polarizing membrane for protection. In the casewhere the protective film is provided on only one surface of themembrane, the aforementioned substrate generally serves as a protectivefilm for the other surface. Since the protective film must have a hightransmittance and a low refringent index (a low retardation value), thecellulose acetate film of the invention is particularly advantageouslyused as the protective film.

As the polarizing membrane, an iodine polarizing membrane, a polyenepolarizing membrane and a dichromatic dye polarizing membrane are known.Those are generally prepared from polyvinyl alcohol films.

The protective film for the polarizing plate has a thickness preferablyin the range of 25 to 350 μm, more preferably in the range of 50 to 200μm. The protective film may contain an ultraviolet absorbent, a slippingagent, a deterioration inhibitor or a plasticizer.

The surface of the protective film may be further covered with a surfacetreatment film, which works for hard coating, anti-fogging andanti-glare.

The polarizing plate and the protective film thereof are described inJapanese Patent Provisional publication Nos. 4(1992)-219703,5(1993)-212828 and 6(1994)-51117.

EXAMPLE 1

(Synthesis of Cellulose Acetate)

Wood pulp (water content: 7.31 wt. %) containing α-cellulose in theamount of about 97 wt. % was broken into pieces. To 302.1 g of the pulp,140 g of glacial acetic acid was evenly sprinkled. The resulting mixturewas then stirred. After left at room temperature for 90 minutes, themixture was poured into another mixture of 769.7 g of cooled aceticanhydride, 1170.3 g of acetic acid and 23.08 g of 98% sulfuric acid. Thetemperature of the obtained mixture was linearly elevated for 60 minutesfrom 0° C. (when the reaction was started) to 37° C. The temperature wasthen kept at 37° C. for 90 minutes, to synthesize cellulose acetate.

(Ripening of Cellulose Acetate)

To a solution of the above-prepared cellulose acetate, 62.05 g of 26 wt.% aqueous solution of acetic acid was added. The mixture was heated to47° C., and kept at the temperature for 90 minutes to ripen thecellulose acetate.

The amounts of acetic acid (acetyl donor), water and sulfuric acid(catalyst) in the mixture were 1,658 weight parts, 23.3 weight parts and22.6 weight parts, respectively, based on 499 weight parts of celluloseacetate. Accordingly, the ratio of water to acetic acid (acetyl donor)was 4.68 mol. %.

The reaction parameter (R=∫YZ/Xdt) in this ripening step was calculatedand found 41.

(Post Treatment)

After the ripening step was completed, 188 g of 24 wt. % aqueoussolution of magnesium acetate was added. The resulting mixture wasstirred, and then the solution was poured while stirred vigorously intoabout 6 L of 10 wt. % aqueous solution of acetic acid. The formedprecipitates were collected by filtration, and then washed successivelywith flowing water, with hot water and again with flowing water. Afterthe solvent was removed with centrifugation, the precipitates were driedat 50° C.

(Analysis of Cellulose Acetate)

With respect to the prepared cellulose acetate, the degree of acetylsubstitution (average in total) and the degree of polymerization weremeasured.

Further, the elution curve of a reverse phase HPLC was measured, andconverted into a distribution curve of intermolecular substitutiondegree to determine the half width of the maximum peak.

The results are set forth in Table 1.

Next, the degrees of substitution at 2-, 3- and 6positions (2DS, 3DS and6DS) were measured. The results are set forth in Table 2. The degrees ofsubstitution at 2-, 3- and 6-positions (2DS, 3DS and 6DS) are plotted asthe solid circle 1 in FIGS. 3 and 4.

The degrees of substitution were measured according to Tezuka,Carbohydr. Res., 273, 83(1995). First, dissociated hydroxyls in thesample (cellulose acetate) were changed into propionate esters withpropionic acid anhydride in pyridine. The obtained sample was thendissolved in heavy chloroform, and a ¹³C-spectrum was measured. Thecarbonyl carbons in the acetyls at 2-, 3- and 6-positions give signalsin the order from higher magnetic field in the range of 169 to 171 ppm.The carbonyl carbons in the propionate esters at 2-, 3- and 6-positionsgive signals in the order from higher magnetic field in the range of 172to 174 ppm. According to the obtained signals, the ratio between theacetyl and the propionyl at 2-, 3- or 6-position was determined toobtain the distribution of acetyls in the sample cellulose acetate.

(Preparation of Cellulose Acetate Solution)

At room temperature, 17 weight parts of prepared cellulose acetate,80.28 weight parts of a mixed solvent of methylacetate/methanol/n-butanol (ratio: 80/15/5 wt. %) and 2.72 weight partsof triphenyl phosphate (plasticizer) were mixed. In the mixture, thecellulose acetate was not dissolved but swelled. The obtained swelledmixture was in the form of slurry.

The swelled mixture was placed in a container having an outer jacket.While the mixture was slowly stirred, water and ethylene glycol asrefrigerant were poured into the outer jacket. The refrigerant cooledthe mixture in the inner container to −30° C. at the cooling rate of 8°C. per minute, and kept cooling for 30 minutes until the mixture washomogeneously solidified.

The refrigerant was then removed from the jacket, and instead hot waterwas poured into the jacket. Becoming in the form of a sol to a degree,the mixture was stirred to warm to room temperature at the warming rateof 8° C. per minute.

The above cooling and warming procedures were repeated.

The solution prepared by the cooling dissolution method was stored atroom temperature (23° C.), and then observed. As a result, even after 20days, the solution kept good transparency and homogeneity, andaccordingly exhibited good solubility and stability.

(Formation of Cellulose Acetate Film)

The prepared solution was cast on a band of 6 m (effective length) toform a film having the thickness of 100 pm. The temperature of the bandwas 0° C. After air was blown for 2 seconds to dry, the film was peeledoff the band. The film was then further dried step by step at 100° C.for 3 minutes, at 130° C. for 5 minutes and at 160° C. for 5 minuteswith the ends of the film fixed, and thereby the solvent remaining inthe film was removed. The prepared film was further dried at 120° C. for3 hours. Thus, a cellulose acetate film was formed.

The formed film exhibited preferable optical characters (high opticalisotropy and transparency).

EXAMPLE 2

(Ripening of Cellulose Acetate)

Commercially available cellulose acetate (polymerization degree: 360,degree of substitution determined by NMR: 2.84), which was obtained byacetylating cotton linter under normal conditions, was used. Thecellulose acetate in the amount of 200 g was dissolved in a mixture of1,167 ml of dichloromethane and 834 ml of acetic acid. From the obtainedsolution, dichloromethane was distilled off by means of a rotaryevaporator. To the resulting liquid, 2,050 g of acetic acid, 2.65 g ofwater and 24.4 g of 70 wt. % aqueous solution of perchloric acid wereadded to dissolve the cellulose acetate. The ratio of water (totalamount of added water and water contained in the aqueous solution ofperchloric acid) to acetic acid (acetyl donor) was 1.63 mol. %.

The obtained solution was kept at 30° C. for 3 hours to ripen thecellulose acetate.

The reaction parameter (R=∫YZ/Xdt) in this ripening step was calculatedand found 55.

(Post Treatment)

After the ripening step was completed, sodium acetate in 2 equivalentweights based on the amount of perchloric acid (1.75 weight parts basedon 1 weight part of 70 wt. % perchloric acid aqueous solution) wasadded. The resulting mixture was stirred well, and then about 7.5 L ofwater was gradually added to the solution with the solution stirredvigorously. The formed precipitates were washed with flowing water untilacetic acid did not smell. After water was removed with centrifugation,the precipitates were further washed with flowing water. Water was againremoved with centrifugation, and then the precipitates were dried at 50°C.

(Analysis of Cellulose Acetate)

The obtained cellulose acetate was analyzed in the same manner as inExample 1. The results are set forth in Tables 1 & 2. The degrees ofsubstitution at 2-, 3- and 6-positions (2DS, 3DS and 6DS) are plotted asthe solid circle 2 in FIGS. 3 & 4.

(Preparation of Cellulose Acetate Solution)

A cellulose acetate solution was prepared in the same manner as inExample 1, except that the obtained cellulose acetate was used.

The prepared solution was stored at room temperature (23° C.), and thenobserved. As a result, even after 20 days, the solution kept goodtransparency and homogeneity, and accordingly exhibited good solubilityand stability.

(Formation of Cellulose Acetate Film)

A cellulose acetate film was formed in the same manner as in Example 1,except that the prepared solution was used.

The formed film exhibited preferable optical characters (high opticalisotropy and transparency).

EXAMPLE 3

(Ripening of Cellulose Acetate)

The commercially available cellulose acetate used in Example 2 in theamount of 200 g was dissolved in a mixture of 1,167 ml ofdichloromethane and 834 ml of acetic acid. From the obtained solution,dichloromethane was distilled off by means of a rotary evaporator. Tothe resulting liquid, 2,050 g of acetic acid, 2.65 g of water and 24.4 gof 70 wt. % aqueous solution of perchloric acid were added to dissolvethe cellulose acetate. The ratio of water (total amount of added waterand water contained in the aqueous solution of perchloric acid) toacetic acid (acetyl donor) was 1.63 mol. %.

The obtained solution was kept at 30° C. for 5 hours to ripen thecellulose acetate.

The reaction parameter (R=∫YZ/Xdt) in this ripening step was calculatedand found 92.

(Analysis of Cellulose Acetate)

The ripened cellulose acetate was treated and analyzed in the samemanner as in. Example 2. The results are set forth in Tables 1 & 2. Thedegrees of substitution at 2-, 3- and 6-positions (2DS, 3DS and 6DS) areplotted as the solid circle 3 in FIGS. 3 & 4.

(Preparation of Cellulose Acetate Solution)

A cellulose acetate solution was prepared in the same manner as inExample 1, except that the obtained cellulose acetate was used.

The prepared solution was stored at room temperature (23° C.), and thenobserved. As a result, even after 20 days, the solution kept goodtransparency and homogeneity, and accordingly exhibited good solubilityand stability.

(Formation of Cellulose Acetate Film)

A cellulose acetate film was formed in the same manner as in Example 1,except that the prepared solution was used.

The formed film exhibited preferable optical characters (high opticalisotropy and transparency).

EXAMPLE 4

(Ripening of Cellulose Acetate)

The commercially available cellulose acetate used in Example 2 in theamount of 200 g was dissolved in a mixture of 1,167 ml ofdichloromethane and 834 ml of acetic acid. From the obtained solution,dichloromethane was distilled off by means of a rotary evaporator. Tothe resulting liquid, 2,050 g of acetic acid, 18.35 g of water and 24.4g of 70 wt. % aqueous solution of perchloric acid were added to dissolvethe cellulose acetate. The ratio of water (total amount of added waterand water contained in the aqueous solution of perchloric acid) toacetic acid (acetyl donor) was 4.18 mol. %.

The obtained solution was kept at 30° C. for 10 hours to ripen thecellulose acetate.

The reaction parameter (R=∫YZ/Xdt) in this ripening step was calculatedand found 72.

(Analysis of Cellulose Acetate)

The ripened cellulose acetate was treated and analyzed in the samemanner as in Example 2. The results are set forth in Tables 1 & 2. Thedegrees of substitution at 2-, 3- and 6-positions (2DS, 3DS and 6DS) areplotted as the solid circle 4 in FIGS. 3 & 4.

(Preparation of Cellulose Acetate Solution)

A cellulose acetate solution was prepared in the same manner as inExample 1, except that the obtained cellulose acetate was used.

The prepared solution was stored at room temperature (23° C.), and then-observed. As a result, even after 20 days, the solution kept goodtransparency and homogeneity, and accordingly exhibited good solubilityand stability.

(Formation of Cellulose Acetate Film)

A cellulose acetate film was formed in the same manner as in Example 1,except that the prepared solution was used.

The formed film exhibited preferable optical characters (high opticalisotropy and transparency).

EXAMPLE 5

(Ripening of Cellulose Acetate)

The commercially available cellulose acetate used in Example 2 in theamount of 200 g was dissolved in a mixture of 1,167 ml ofdichloromethane and 834 ml of acetic acid. From the obtained solution,dichloromethane was distilled off by means of a rotary evaporator. Tothe resulting liquid, 2,050 g of acetic acid, 18.35 g of water and 24.4g of 70 wt. % aqueous solution of perchloric acid were added to dissolvethe cellulose acetate. The ratio of water (total amount of added waterand water contained in the aqueous solution of perchloric acid) toacetic acid (acetyl donor) was 4.18 mol. %.

The obtained solution was kept at 30° C. for 15 hours to ripen thecellulose acetate.

The reaction parameter (R=∫YZ/Xdt) in this ripening step was calculatedand found 107.

(Analysis of Cellulose Acetate)

The ripened cellulose acetate was treated and analyzed in the samemanner as in Example 2. The results are set forth in Tables 1 & 2. Thedegrees of substitution at 2-, 3- and 6-positions (2DS, 3DS and 6DS) areplotted as the solid circle 5 in FIGS. 3 & 4.

(Preparation of Cellulose Acetate Solution)

A cellulose acetate solution was prepared in the same manner as inExample 1, except that the obtained cellulose acetate was used.

The prepared solution was stored at room temperature (23° C.), and thenobserved. As a result, even after 20 days, the solution kept goodtransparency and homogeneity, and accordingly exhibited good solubilityand stability.

(Formation of Cellulose Acetate Film)

A cellulose acetate film was formed in the same manner as in Example 1,except that the prepared solution was used.

The formed film exhibited preferable optical characters (high opticalisotropy and transparency).

COMPARISON EXAMPLE 1

(Ripening of Cellulose Acetate)

The cellulose acetate prepared in Example 1 in the amount of 200 g wasdissolved in a mixture of 1,167 ml of dichloromethane and 834 ml ofacetic acid. From the obtained solution, dichloromethane was distilledoff by means of a rotary evaporator. To the resulting liquid, 2,050 g ofacetic acid, 54.13 g of water and 24.4 g of 70 wt. % aqueous solution ofperchloric acid were added to dissolve the cellulose acetate. The ratioof water (total amount of added water and water contained in the aqueoussolution of perchloric acid) to acetic acid (acetyl donor) was 10.00mol. %.

The obtained solution was kept at 30° C. for 20 hours to ripen thecellulose acetate.

The reaction parameter (R=∫YZ/Xdt) in this ripening step was calculatedand found 60.

(Analysis of Cellulose Acetate)

The ripened cellulose acetate was treated and analyzed in the samemanner as in Example 2. The results are set forth in Tables 1 & 2. Thedegrees of substitution at 2-, 3- and 6-positions (2DS, 3DS and 6DS) areplotted as the open circle C1 in FIGS. 3 & 4.

(Preparation of Cellulose Acetate Solution)

A cellulose acetate solution was prepared in the same manner as inExample 1, except that the obtained cellulose acetate was used.

The prepared solution was stored at room temperature (23° C.), and thenobserved. As a result, the solution showed white turbidity, and opaquelump was observed.

COMPARISON EXAMPLE 2

(Ripening of Cellulose Acetate)

The cellulose acetate prepared in Example 1 in the amount of 200 g wasdissolved in a mixture of 1,167 ml of dichloromethane and 834 ml ofacetic acid. From the obtained solution, dichloromethane was distilledoff by means of a rotary evaporator. To the resulting liquid, 2,050 g ofacetic acid, 54.13 g of water and 24.4 g of 70 wt. % aqueous solution ofperchloric acid were added to dissolve the cellulose acetate. The ratioof water (total amount of added water and water contained in the aqueoussolution of perchloric acid) to acetic acid (acetyl donor) was 10.00mol. %.

The obtained solution was kept at 30° C. for 30 hours to ripen thecellulose acetate.

The reaction parameter (R=∫YZ/Xdt) in this ripening step was calculatedand found 90.

(Analysis of Cellulose Acetate)

The ripened cellulose acetate was treated in the same manner as inExample 1.

Next, the degrees of substitution at 2-, 3- and 6positions (2DS, 3DS and6DS) were measured. The results are set forth in Table 2. The degrees ofsubstitution at 2-, 3- and 6-positions (2DS, 3DS and 6DS) are plotted asthe solid circle 1 in FIG. 4 (while the open circle C2 is out of therange shown in FIG. 3).

(Preparation of Cellulose Acetate Solution)

A cellulose acetate solution was prepared in the same manner as inExample 1, except that the obtained cellulose acetate was used.

The prepared solution was stored at room temperature (23° C.), and thenobserved. As a result, the solution was transparent, but partially notuniform.

COMPARISON EXAMPLE 3

(Synthesis of Cellulose Acetate)

To 100 weight parts of cellulose (made from wood pulp), 14.2 weightparts of sulfuric acid, 260 weight parts of acetic anhydride and 360weight parts of acetic acid were added. The mixture was subjected to anacetylation reaction at 40° C. for 95 minutes. Two third of sulfuricacid was neutralized with magnesium acetate to form magnesium sulfate.

The amount of water to acetic acid (acetyl donor) after neutralizingreaction was 20 mol %.

(Ripening of Cellulose Acetate)

The obtained solution was kept at 65° C. for 100 minutes to ripencellulose acetate. The solution was treated in the same manner as inExample 1, and cellulose acetate was separated from the solution.

The reaction parameter (R=∫YZ/Xdt) in this ripening step was calculatedand found 15.

(Analysis of Cellulose Acetate)

The ripened cellulose acetate was analyzed in the same manner as inExample 1. The results are set forth in Tables 1 & 2.

(Preparation of Cellulose Acetate Solution)

A cellulose acetate solution was prepared in the same manner as inExample 1, except that the obtained cellulose acetate was used.

The prepared solution was stored at room temperature (23° C.), and thenobserved. As a result, the solution was transparent, but partially notuniform.

COMPARISON EXAMPLE 4

(Synthesis of Cellulose Acetate)

To 550 g of cotton linter, 2747 g of acetic acid, 1630 g of aceticanhydride and 64.4 g of sulfuric acid were added in a kneader. Themixture was heated from 0° C. to 40° C. for 120 minutes to conductacetylation reaction.

(Ripening of Cellulose Acetate)

To the obtained solution, 328 g of 24 wt. % aqueous solution ofmagnesium acetate was added. The resulting solution (in consideration ofwater contents of stating materials) comprised 979 g of celluloseacetate, 4053 g of acetic acid, 10.1 g sulfuric acid and 177 g of water.The ratio of water/acetyl donor was 14.5 mol %.

The obtained solution was kept at 52° C. for 100 minutes to ripencellulose acetate.

The reaction parameter (R=∫YZ/Xdt) in this ripening step was calculatedand found 3.7.

(Post Treatment)

After ripening cellulose acetate, 376 g of 24 wt. % aqueous solution ofmagnesium acetate was added to the cellulose acetate, and the mixturewas stirred. The obtained solution was poured into about 12 liter of 10wt. % aqueous solution of acetic acid while stirring vigorously. Theformed precipitates were filtered off, washed with water flow, washedwith hot water, washed with water flow again, centrifuged from liquid,and dried at 50° C.

(Analysis of Cellulose Acetate)

With respect to the prepared cellulose acetate, the degree of acetylsubstitution (average in total) and the degree of polymerization weremeasured.

Further, the elution curve of a reverse phase HPLC was measured, andconverted into a distribution curve of intermolecular substitutiondegree to determine the half width of the maximum peak.

The results are set forth in Table 1.

TABLE 1 Total Maximum peak Degree Cellu- degree Degree of poly- lose ofsub- of sub- Half Value meriza- Solubi- acetate stitution stitution¹⁾Width of Y²⁾ tion lity³⁾ Ex. 1 2.854 2.847 0.066 0.069 284 A Ex. 2 2.8542.854 0.067 0.069 302 A Ex. 3 2.856 2.853 0.059 0.068 284 A Ex. 4 2.7862.795 0.067 0.081 305 A Ex. 5 2.765 2.811 0.071 0.084 291 A Comp. 12.684 2.692 0.100 0.097 350 C Comp. 3 2.733 2.737 0.096 0.089 292 BComp. 4 2.844 2.851 0.086 0.070 298 B (Remarks) ¹⁾Degree of substitutionat the maximum peak ²⁾Y = −0.1788X + 0.5788 (wherein X is the totaldegree of substitution) ³⁾A: Transparent uniform solution B: Transparentbut not uniform C: White, turbid and not transparent

TABLE 2 Cellu- Amount Reaction Degree of substitution lose of waterparameter of cellulose acetate Degree of acetate (mol %) (R) 2DS 3DS 6DSpolymerization Ex. 1 4.68 41 0.959 0.955 0.940 284 Ex. 2 1.63 55 0.9620.960 0.932 302 Ex. 3 1.63 92 0.960 0.953 0.943 284 Ex. 4 4.18 72 0.9370.917 0.932 305 Ex. 5 4.18 107 0.929 0.896 0.940 291 Comp. 1 10.00 600.917 0.869 0.898 350 Comp. 2 10.00 90 0.882 0.826 0.907 339 Comp. 319.67 15 0.947 0.947 0.839 292

EXAMPLE 6

(Synthesis of Cellulose Acetate)

To 100 weight parts of cellulose prepared from cotton linter, 9.2 weightparts of sulfuric acid, 276 weight parts of acetic anhydride and 551weight parts of acetic acid were added. The cellulose was then subjectedto an ester forming reaction in a conventional manner. After neutralizedwith magnesium acetate, the reaction mixture was kept at 62° C. for 40minutes to prepare cellulose acetate.

(Ripening of Cellulose Acetate)

The obtained cellulose acetate was ripened in the manner described inExample 1 except that the ratios of water and perchloric acid (catalyst)to acetic acid (acetyl donor) and the reaction parameter (R=∫YZ/Xdt)were changed into 1.63 mol. %, 0.498 mol. % and 55, respectively.

(Analysis of Cellulose Acetate)

The ripened cellulose acetate was analyzed in the same manner as inExample 1. The results are set forth in Table 3. The degrees ofsubstitution at 2-, 3- and 6-positions (2DS, 3DS and 6DS) are plotted asthe solid circle 6 in FIGS. 3 & 4.

(Measurement of Infrared Absorption Spectrum)

In 5 g of a mixed solvent of methylene chloride/methanol (9/1, byweight), 200 mg of the cellulose acetate dried well was dissolved. Thesolution was cast on a glass plate by means of a bar coater so that thethickness might be even. The solution layer on the plate was dried byair to form a film. The thickness of the film was adjusted so that thetransmittance (at the peak attributed to non-substituted hydroxyl of thecellulose acetate) might be about 40%.

The formed film was dried in vacuum at 105° C. for 30 minutes or more,and then cut into pieces having enough sizes (24 mm×27 mm) for a flameof IR generator so that IR rays might not be interrupted by a sampleholder.

The film sample was kept under nitrogen atmosphere until the absorptionpeak at about 3650 cm⁻¹ attributed to adsorbed water disappeared. Theinfrared absorption spectrum was measured by means of FT-IR1650(Parkin-Elmer). The measured spectrum was analyzed in the mannerdescribed above to obtain a half-width (Δν_(1/2)). The results are setforth in Table 3.

EXAMPLE 7

(Ripening of Cellulose Acetate)

Commercially available cellulose acetate (polymerization degree: 360,degree of substitution determined by NMR: 2.84), which was obtained byacetylating cotton linter under normal conditions, was used. Thecellulose acetate in the amount of 1,150 weight parts was dissolved in amixture of 8,220 weight parts of dichloromethane and 4,800 weight partsof acetic acid. From the obtained solution, dichloromethane wasdistilled off by means of a rotary evaporator. The obtained liquid wastransferred into a reaction tank, and 11,788 weight parts of aceticacid, 78 weight parts of water and 98 weight parts of perchloric acidwere added. The ratio of water to acetic acid (acetyl donor) was 2.2mol. %.

The obtained solution was kept at 30° C for 5 hours to ripen thecellulose acetate.

The reaction parameter (R=∫YZ/Xdt) in this ripening step was calculatedand found 67.5.

(Post Treatment)

After the ripening step was completed, 1.75 equivalent weights (based onthe amount of perchloric acid) of sodium acetate in 10 wt. % acetic acidsolution was added. The resulting mixture was stirred for 10 minutes tostop the reaction. Thus, cellulose acetate (viscosity average molecularweight: 288, degree of substitution: 2.84) was prepared.

(Analysis of Cellulose Acetate)

The obtained cellulose acetate was analyzed in the same manner as inExample 1. The results are set forth in Table 3. The degrees ofsubstitution at 2-, 3- and 6- positions (2DS, 3DS and 6DS) are plottedas the solid circle 7 in FIGS. 3 & 4.

(Measurement of Infrared Absorption Spectrum)

The infrared absorption spectrum of the cellulose acetate was measuredin the same manner as in Example 6 to obtain a half-width (Δν_(1/2)).The results are set forth in Table 3.

(Preparation of Cellulose Acetate Solution)

The prepared cellulose acetate in the amount of 15 weight parts wasmixed in room temperature (25° C.) with 68 weight parts of methylacetate and 17 weight parts of acetone, to prepare a slurry of 15 wt. %.The slurry was almost in the form of a transparent gel, but some opaquejellied parts were observed.

The slurry was cooled for 2 hours in cold methanol bath beforehandcooled at −40° C. with dry ice. In the thus cooled slurry, many bubbleswere formed by penetration of the solvent. After left at roomtemperature for 10 minutes, the slurry was kept for 10 minutes in waterbath at 40° C. The thus obtained cellulose acetate solution had goodtransparency and fluidity.

EXAMPLE 8

(Ripening of Cellulose Acetate)

Commercially available cellulose acetate (polymerization degree: 360,degree of substitution determined by NMR: 2.84), which was obtained byacetylating cotton linter under normal conditions, was used. Thecellulose acetate in the amount of 200 weight parts was dissolved in amixture of 1,600 weight parts of dichloromethane and 867 weight parts ofacetic acid. From the obtained solution, dichloromethane was distilledoff by means of a rotary evaporator. The obtained liquid was transferredinto a reaction tank, and 2,050 weight parts of acetic acid, 18.7 weightparts of water and 24.3 weight parts of 70 wt. % aqueous solution ofperchloric acid were added. The ratio of water to acetic acid (acetyldonor) was 4.2 mol. %.

The obtained solution was kept at 30° C. for 10 hours to ripen thecellulose acetate.

The reaction parameter (R=∫YZ/Xdt) in this ripening step was calculatedand found 71.

(Post Treatment)

After the ripening step was completed, 1.75 equivalent weights (based onthe amount of perchloric acid) of sodium acetate in 10 wt. % acetic acidsolution was added. The resulting mixture was stirred for 10 minutes tostop the reaction. Thus, cellulose acetate (viscosity average molecularweight: 318, degree of substitution: 2.81) was prepared.

(Analysis of Cellulose Acetate)

The obtained cellulose acetate was analyzed in the same manner as inExample 1. The results are set forth in Table 3. The degrees ofsubstitution at 2-, 3- and 6-positions (2DS, 3DS and 6DS) are plotted asthe solid circle 8 in FIGS. 3 & 4.

(Measurement of Infrared Absorption Spectrum)

The infrared absorption spectrum of the cellulose acetate was measuredin the same manner as in Example 6 to obtain a half-width (Δν_(1/2)).The results are set forth in Table 3.

(Preparation of Cellulose Acetate Solution)

The prepared cellulose acetate in the amount of 15 weight parts wasmixed in room temperature (25° C.) with 68 weight parts of methylacetate and 17 weight parts of acetone, to prepare a slurry of 15 wt. %.The slurry was almost in the form of a transparent gel, but some opaquejellied parts were observed.

The slurry was cooled for 2 hours in cold methanol bath beforehandcooled at −40° C. with dry ice. In the thus cooled slurry, many bubbleswere formed by penetration of the solvent. After left at roomtemperature for 10 minutes, the slurry was kept for 10 minutes in waterbath at 40° C. The thus obtained cellulose acetate solution had goodtransparency and fluidity.

EXAMPLE 9

(Ripening of Cellulose Acetate)

Commercially available cellulose acetate (polymerization degree: 360,degree of substitution determined by NMR: 2.84), which was obtained byacetylating cotton linter under normal conditions, was used. Thecellulose acetate was ripened in the manner described in Example 1except that the time for ripening, the ratios of water and perchloricacid (catalyst) to acetic acid (acetyl donor) and the reaction parameter(R=∫YZ/Xdt) were changed into 15 hours, 4.18 mol. %, 0.498 mol. % and107, respectively.

(Analysis of Cellulose Acetate)

The obtained cellulose acetate was analyzed in the same manner as inExample 1. The results are set forth in Table 3. The degrees ofsubstitution at 2-, 3- and 6- positions (2DS, 3DS and 6DS) are plottedas the solid circle 9 in FIGS. 3 & 4.

(Measurement of Infrared Absorption Spectrum)

The infrared absorption spectrum of the cellulose acetate was measuredin the same manner as in Example 6 to obtain a half-width (Δν_(1/2)).The results are set forth in Table 3.

TABLE 3 Reac- tion Reac- Degree of substitution Degree Amount para- tionHalf of cellulose of of water meter time Width acetate polymeri- Ex.(mol %) (R) ¹⁾ ²⁾ 2DS 3DS 6DS zation 6 1.63 55 3 128 0.965 0.963 0.933300 7 2.20 67.5 5 123 0.959 0.943 0.937 288 8 4.18 71 10 121 0.945 0.9260.935 318 9 4.18 107 15 119 0.925 0.896 0.941 301 (Remarks) ¹⁾Time forripening cellulose acetate (hours), ²⁾Half-width (cm⁻¹) of theabsorption peak in the region of 3450 to 3550 cm⁻¹

EXAMPLE 10

(Measurement of Infrared Absorption Spectrum)

With respect to cellulose acetate (total degree of substitution2DS+3DS+6DS: 2.896, 2DS+3DS-6DS: 0.896), the infrared absorptionspectrum was measured in the same manner as in Example 6. The obtainedspectrum is shown in FIG. 6, and the half-width (Δν_(1/2)) was found 90.

EXAMPLE 11

(Measurement of Infrared Absorption Spectrum)

With respect to cellulose acetate (total degree of substitution2DS+3DS+6DS: 2.877, 2DS+3DS-6DS: 0.967), the infrared absorptionspectrum was measured in the same manner as in Example 6. The obtainedspectrum is shown in FIG. 6, and the half-width (Δν_(1/2)) was found118.

COMPARISON EXAMPLE 5

(Measurement of Infrared Absorption Spectrum)

With respect to cellulose acetate (total degree of substitution2DS+3DS+6DS: 2.845, 2DS+3DS-6DS: 1.069), the infrared absorptionspectrum was measured in the same manner as in Example 6. The obtainedspectrum is shown in FIG. 6, and the half-width (Δν_(1/2)) was found137.

COMPARISON EXAMPLE 6

(Measurement of Infrared Absorption Spectrum)

With respect to cellulose acetate (total degree of substitution2DS+3DS+6DS: 2.925, 2DS+3DS-6DS: 1.075), the infrared absorptionspectrum was measured in the same manner as in Example 6. The obtainedspectrum is shown in FIG. 6, and the half-width (Δν_(1/2)) was found137.

EXAMPLE 12

(Ripening of Cellulose Acetate)

A dried commercially available cellulose acetate (polymerization degree:311, degree of substitution determined by NMR: 2.85), which was obtainedby acetylating wood pulp under normal conditions, was obtained. Thecellulose acetate in the amount of 500 g was dissolved in a mixture of1,333 g of dichloromethane and 1,033 g of acetic acid. With the solutionstirred, 150 g of acetic acid containing 13.3 g of water was added.After homogenized, the solution was kept at 40° C. While the solutionwas stirred, 150 g of acetic acid containing 36.92 g of toluenesulfonicacid monohydrate was added to start ripening. The ratio of water toacetic acid (acetyl donor) was 4.2 mol. %. After 7 hours, 260 g ofacetic acid containing 29 g of sodium acetic anhydride was dropped tostop ripening.

The reaction parameter (R=∫YZ/Xdt) in this ripening step was calculatedand found 151.

(Post Treatment)

From the reacted solution, dichloromethane was distilled off by means ofa rotary evaporator. To the resulting solution, about 4.5 L of water wasgradually added with the solution stirred vigorously in the same manneras in Example 2. The formed precipitates were collected, washed, and thesolvent was removed by centrifugation. The precipitates were furtherwashed with flowing water, and the solvent was removed again bycentrifugation. The precipitates were then dried at 50° C.

(Analysis of Cellulose Acetate)

The obtained cellulose acetate was analyzed in the same manner as inExample 1. The results are set forth in Table 4. The degrees ofsubstitution at 2-, 3- and 6-positions (2DS, 3DS and 6DS) are plotted asthe solid circle 12 in FIGS. 3 & 4.

EXAMPLE 13

(Ripening of Cellulose Acetate)

The cellulose acetate was ripened and subjected to post treatment in themanner described in Example 12 except that the amount of water (theratio of water to acetic acid (acetyl donor)) and the reaction time werechanged into 2.91 g (1.6 mol. %) and 140 minutes, respectively.

The reaction parameter (R=∫YZ/Xdt) in this ripening step was calculatedand found 132.

(Analysis of Cellulose Acetate)

The ripened cellulose acetate was analyzed in the same manner as inExample 1. The results are set forth in Table 4. The degrees ofsubstitution at 2-, 3- and 6-positions (2DS, 3DS and 6DS) are plotted asthe solid circle 13 in FIGS. 3 & 4.

EXAMPLE 14

(Synthesis of Cellulose Acetate)

Wood pulp (water content: 8.2 wt. %) was broken into pieces. To 1,520 gof the pulp, 698 g of acetic acid was evenly sprinkled. The resultingmixture was then stirred. After left at room temperature for 90 minutes,the mixture was poured into another mixture of 3930.6 g of aceticanhydride beforehand cooled at about −10° C., 5755 g of acetic acid and115.4 g of 98% sulfuric acid. The temperature of the obtained mixturewas linearly elevated for 70 minutes from 0° C. (when the reaction wasstarted) to 37° C. The temperature was then kept at 37° C. for 80minutes, to synthesize cellulose acetate.

(Ripening of Cellulose Acetate)

To a solution of the above-prepared cellulose acetate, 383.7 g of 30%aqueous solution of acetic acid was added. The mixture was heated to 47°C., and kept at the temperature for 130 minute. The ratio of water toacetic acid (acetyl donor) was 6.0 mol. %. After that, 297 g of aqueoussolution of magnesium acetate tetrahydrate was added to stop ripening.

The reaction parameter (R=∫YZ/Xdt) in this ripening step was calculatedand found 43.

(Post Treatment)

The resulting solution was poured into about 30 L of 10% acetic acidaqueous solution with the solution stirred vigorously. The formedprecipitates were collected, washed successively with flowing water,with hot water and again with flowing water, and then the solvent wasremoved by centrifugation. The precipitates were then dried at 50° C.

(Analysis of Cellulose Acetate)

The obtained cellulose acetate was analyzed in the same manner as inExample 1. The results are set forth in Table 4. The degrees ofsubstitution at 2-, 3- and 6-positions (2DS, 3DS and 6DS) are plotted asthe solid circle 14 in FIGS. 3 & 4.

TABLE 4 Amount Reaction Degree of substitution Degree of Cellulose ofwater parameter of cellulose acetate poly- acetate (mol %) (R) 2DS 3DS6DS merization Ex. 12 4.20 151 0.953 0.936 0.925 288 Ex. 13 1.60 1320.972 0.967 0.916 298 Ex. 14 6.00 43 0.973 0.967 0.923 303

1. A process for preparation of cellulose acetate having a degree ofacetyl substitution in the range of 2.636 to 2.958 wherein the degree ofacetyl substitution at the 2-, 3-, and 6- positions satisfy thefollowing formula (I) which comprises the steps of: reacting cellulosein a solvent with acetic acid and acetic anhydride in the presence of anacid catalyst to synthesize cellulose acetate; and ripening thesynthesized cellulose acetate in the presence of the remaining acidcatalyst, an acetyl donor, and water or an alcohol at a temperature of30 to 70° C., and under a condition that the amount of water and thealcohol is in the range of 0.1 to 10 mol % based on the amount of theacetyl donor in the ripening step:2DS+3DS<6DSx4−1.70  (I) in which 2DS is the degree of acetylsubstitution at 2-position; 3DS is the degree of acetyl substitution at3-position; and 6DS is the degree of acetyl substitution at 6-position.2. The process for preparation of cellulose acetate according to claim1, wherein the amount of said water and alcohol is 0.1 to 7 mol %. 3.The process for preparation of cellulose acetate according to claim 1,wherein said acid catalyst is selected from sulfonic acid, perchloricacid, sulfuric acid, boron trifluoride or tetrafluroboric acid.
 4. Aprocess for preparation of cellulose acetate having a degree of acetylsubstitution in the range of 2.636 to 2.958 wherein the degree of acetylsubstitution at the 2-, 3-, and 6- positions satisfy the followingformula (I) which comprises the steps of: reacting cellulose in asolvent with acetic acid and acetic anhydride in the presence of an acidcatalyst to synthesize cellulose acetate; neutralizing the acid catalystto stop the synthesizing reaction; and ripening the synthesizedcellulose acetate in the presence of a catalyst, an acetyl donor, andwater or an alcohol at a temperature of 30 to 70° C., and under acondition that the amount of water and the alcohol is in the range of0.1 to 10 mol % based on the amount of the acetyl donor in the ripeningstep:2DS+3DS<6DSx4−1.70  (I) in which 2DS is the degree of acetylsubstitution at 2-position; 3DS is the degree of acetyl substitution at3-position; and 6DS is the degree of acetyl substitution at 6-position.5. A process for adjusting an intramolecular degree of acetylsubstitution of cellulose acetate to obtain cellulose acetate in whichthe degrees of acetyl substitution at 2-, 3- and 6-positions satisfy thefollowing formulas (I), (III) and (IV), which comprises: ripeningcellulose acetate in the presence of a catalyst, an acetyl donor, andwater or an alcohol at a temperature of 30 to 70° C., and under acondition that the amount of water and the alcohol is in the range of0.1 to 10 mol % based on the amount of acetyl donor in the ripeningstep:2DS+3DS<6DSx4−1.70  (I)2DS+3DS>1.80  (III)3DS<2DS  (IV) in which 2DS is the degree of acetyl substitution at2-position, 3DS is the degree of acetyl substitution at 3-position, and6DS is the degree of acetyl substitution at 6-position.
 6. A process forpreparation of cellulose acetate in which the degrees of acetylsubstitution at 2-, 3- and 6-positions satisfy the following formulas(I), (III) and (IV), which comprises the steps of: reacting cellulose ina solvent with acetic acid and acetic anhydride in the presence of anacid catalyst to synthesize cellulose acetate; and ripening thesynthesized cellulose acetate in the presence of the remaining acidcatalyst, an acetyl donor, and water or an alcohol at a temperature of30 to 70° C., and under a condition that the amount of water and thealcohol is in the range of 0.1 to 10 mol % based on the amount of theacetyl donor in the ripening step:2DS+3DS<6DSx4−1.70  (I)2DS+3DS>1.80  (III)3DS<2DS  (IV) in which 2DS is the degree of acetyl substitution at2-position, 3DS is the degree of acetyl substitution at 3-position, and6DS is the degree of acetyl substitution at 6-position.
 7. A process forpreparation of cellulose acetate in which the degrees of acetylsubstitution at 2-, 3- and 6-positions satisfy the following formulas(I), (III) and (IV), which comprises the steps of: reacting cellulose ina solvent with acetic acid and acetic anhydride in the presence of anacid catalyst to synthesize cellulose acetate; neutralizing the acidcatalyst to stop the synthesizing reaction; and ripening the synthesizedcellulose acetate in the presence of catalyst, an acetyl donor, andwater or an alcohol at a temperature of 30 to 70° C., and under acondition that the amount of water and the alcohol is in the range of0.1 to 10 mol % based on the amount of the acetyl donor in the ripeningstep:2DS+3DS<6DSx4−1.70  (I)2DS+3DS>1.80  (III)3DS<2DS  (IV) in which 2DS is the degree of acetyl substitution at2-position, 3DS is the degree of acetyl substitution at 3-position, and6DS is the degree of acetyl substitution at 6-position.